A traditional camera lens generates images with varying degrees of defocusing and light fall-off to create a depth of field in an image. A depth of field is the area of sharpness from near to far within a given scene in a photograph. The depth of field of an image is controlled by adjusting the focal length of the camera lens, or zoom factor, and the size of the aperture opening, or f-stop setting, of the camera lens. The size of the aperture opening determines the amount of light allowed into a camera lens to expose the film. For example, as the f-stop increases and the aperture hole becomes smaller, objects in the scene that are farther away from the camera become focused. Likewise, as the f-stop decreases and the aperture hole becomes larger, these same objects become defocused. As the aperture hole gets smaller, less light enters the camera, thus darkening the image. This is referred to as light fall-off.
Traditional computer graphics systems cannot simulate defocusing and light fall-off effects similar to those generated by a camera lens for an image in two dimensional space. One approach attempts to apply a shading effect to two dimensional images to simulate depth of field. This approach employs hardware to alter the lighting applied to an image and does not model a camera depth of field effect. Other approaches apply a crude blurring effect to the image without reference to traditional camera operation